Plan for the coming winter

Laurie Litman

The mysterious Oak Mortality Syndrome (also known as Sudden Oak Death) recently became a little less mysterious as researchers identifed a fungus in the genus Phytopthera as the probable culprit in the case. Although more questions than answers remain, this is a giant step forward toward an eventual understanding of the disease.
Oak Mortality Syndrome was first observed in Marin County in 1995 where seemingly healthy tanoak trees (Lithocarpus densiflorus) suddenly turned brown and died in a matter of weeks. Symptoms included dark, oozing sap and a rapid wilting and browning of leaves. A number of pests were associated with diseased trees: various bark beetles and a fungus of the Hypoxylon genus were invariably found. Theories regarding the cause of the disease ranged from a single pathogen to a pest complex or stress response in the trees.

Since tanoak has little economic value and is considered by many an undesirable species, early reports of the die-back did not cause much concern. However, as the rate of die-back increased and moved into other counties, and as other species such as coastal live oak (Quercus agrifolia) and black oak (Quercus kelloggii) were also found to be infected, people began to take notice.

The new fungus is an unknown, as yet unnamed, species of Phytopthera, a group that is responsible for many plant diseases. (Another infamous member of that group was responsible for the potato blight that devastated Europe.)

Fungi are unlike higher plants in that they lack chlorophyll and can’t produce their own food. Instead, they feed on substances drawn from living or dead plants or animals. This newly identified Phytopthera releases enzymes that break down cells in the tree, causing the red oozing symptoms. Eventually, nutrients are unable to travel between the roots and crown, resulting in brown leaves and tree death.

At this time, it appears that the bark beetles and other species of fungi associated with the diseased trees may simply act as secondary pests, that is, they come in after the trees are weakened by the Phytopthera. While not the primary pathogen, these secondary pests cause additional damage and may hasten tree death.

We still have much to learn before adequate measures are found to respond to this disease. One basic question that needs to be answered is how the disease spreads. It is known that Phytopthera species generally move through wood, soil, and water. Researchers are concerned that the disease may be transported in soil on the tires of cars and bikes, the shoes of hikers, by animals, etc. Preventing the movement of soil and wood from infected areas is a difficult undertaking but may be critical to slowing the spread of the disease.

We also need to learn some basic facts about the disease: what species of trees are susceptible, which areas are at risk, what kinds of resistance exists, what role the beetles play in transport and tree death, how environmental factors may play a role, and, most importantly, what kind of management options exist to deal with this disease.

While the present situation is serious, the potential long-term consequences of Oak Mortality Syndrome are even more grave.

Oaks provide vital habitat and food for many species of animals—deer, birds, rodents, invertebrates—including many sensitive species. The death of large numbers of trees can have far-reaching effects on these communities.

There is also a great deal of concern about the fire consequences of the disease. The die off of large numbers of trees—up to 90 percent of affected species in some areas of Marin County—will add additional fuel to the forests in the form of dead and decaying trees. Worse yet is the possibility that the forest communities will change in such a way as to become more flammable. Oaks tend to be fire resistant when healthy and can be counted on to slow a wildfire. In fact, fire control efforts often take advantage of this characteristic of hardwood stands. If the hardwood forest is devastated by the die-back, these forests may be transformed by the colonization of more flammable species. There is speculation that this conversion of hardwood forests could change fire behavior and increase wildfire damage well into the next century.

At this time there is no known control for this disease. Researchers are pursuing a number of areas including the development of a fungicide to combat this pathogen, but in the meantime, the best recommendation is to keep oaks healthy and try to mitigate any stress to the trees. The California Oak Foundation has information on the care of oaks (see website address in the box below). Any dead trees should be cut down and the logs covered with clear plastic sheets to prevent emergence of beetles.

Visit these sites for background and research updates:

California Oak Mortality Task Force
http://nature.berkeley.edu/comtf/

UC Cooperative Extention in Marin Sudden Oak Death site
http://cemarin.ucdavis.edu/index2.html

UC Integrated Hardwood Range Management Program (IHRMP)
http://danr.ucop.edu/ihrmp/

CAMFER
http://camfer.CNR.Berkeley.EDU/oaks/

Greenbrae.org
http://www.greenbrae.org/

UC IPM Online
http://www.ipm.ucdavis.edu/

California Oak Foundation
http://www.californiaoaks.org/